Sihler, A.
New Comparative Grammar of Greek and Latin.
Oxford University Press.
2008.

ser
(Verb)
"to be"
10th cent. Old Spanish seer. A collapse of Vulgar Latin sedere "to sit" and essere "to be" into a single verb. Already by the 4th cent., Iberian Vulgar Latin sedere was employed in the manner of Modern Spanish ser and not in the sense of sitting. For the etymology of sedere, see notes under sentar. Essere is from Latin esse 'id.'
From Proto-Italic *es- 'id.' From Proto-Indo-European *h1es- 'id.'
Indo-European